In the field of relief valve design for tanks under positive or negative pressure relative to the atmosphere, it has always been necessary to have devices to relieve excess pressure or eliminate a vacuum or negative pressure condition. In this field devices of all types have been employed. Simple weight-loaded disc valves have been used for many years but have the disadvantage of leakage occurring at pressures significantly below the relieving pressure. For example, a weight loaded disc valve over a vent has been provided which initially opens at a predetermined pressure. However, these devices normally require a 50 to 100 percent overpressure above the set pressure to fully lift the valve member and reach a full open position at the full rated capacity. As a result such valves heretofore have required a low set pressure relative to the maximum allowable working pressure (MAWP) of the pressure vessel which requires that normal operating pressure be kept at low inefficient levels, or else the valves have a set pressure closer to MAWP but will not obtain full rated capacity at 20 percent overpressure. In order to avoid the above undesirable conditions, a larger size vent than normally required is sometimes utilized. The required capacity at a 20 percent overpressure above MAWP as required by certain regulations is obtained at a partial lift. However, utilizing a larger valve than needed is uneconomical. More complex pressure relieving devices have also been employed such as pilot operated relief valves. Pilot operated relief valves are used when very tight seating and low overpressure requirements to full capacity are desirable.
As shown in pending application Ser No. 906,559, a safety relief valve is provided for an emergency vent in a pressure vessel particularly adapted for use with low pressure vessels designed for a pressure under 15 psi. A weighted valve member is seated on a seat ring under a predetermined constant loading and is moved upwardly to an unseated open position at a predetermined fluid pressure within the pressure vessel above the maximum allowable working pressure (MAWP). An operating rod is secured adjacent its upper end to the weighted valve member and secured adjacent its lower end to a diaphragm. A separate pilot valve is in fluid communication with a diaphragm fluid chamber formed by the operating rod diaphragm thereby to control the operation of the main valve member. Thus, a pilot valve separate from the pressure relief valve is provided for control of the main valve member.